Magnetostrictive alloys change in dimension in response to an applied magnetic field, and have been used in sonar transducers, actuators, vibration control and sensors. Of particular interest are alloys of iron and rare earth elements that have large magnetostriction constants. Examples of such materials are disclosed in U.S. Pat. No. 4,308,474 to Savage et al., which is hereby incorporated by reference. These include alloys of iron, usually with terbium (Tb) and dysprosium (Dy). The alloys that have to date shown the best magnetostrictive properties are known as "Terfenol-D" and have the general formula Tb.sub.x Dy.sub.1-x Fe.sub.2-w, where 0.20.ltoreq.x.ltoreq.1.00 and 0.ltoreq.w.ltoreq.0.20.
Terfenol-D and other similar alloys, because of their high magnetostriction constants, are of particular interest for use in micro-mechanical systems, such as sensors and actuators. Shapes, such as rods, can be made by melting or casting to form a polycrystalline product. A problem with these shapes is that they are particularly brittle, which severely limits their applicability. Furthermore, the pure magnetostrictive Terfenol-D like alloys are difficult or impossible to machine, which precludes shapes that cannot be cast or molded.
In order to increase the toughness of shapes of Terfenol-D and like magnetostrictive materials, it has been proposed to form a composite of the magnetostrictive material in a binding matrix of another material. For example, Terfenol-D alloys with a slightly raised rare earth (where w is greater than about 0.1) can be hot-pressed into a composite of a ductile Dy-Tb solid solution between a brittle Tb.sub.x Dy.sub.1-x Fe.sub.2 phase. Dy and Tb have high tensile ductilities, up to 20%. While there is an improvement in toughness, the improvement is only modest and far below what might be expected based upon the high ductility of the rare earth phase. The problem is in the microstructure of the phase binding the Tb.sub.x Dy.sub.1-x Fe.sub.2 particles together. The heat required in the formation of the shape results in formation of a proeutectic phase of the rare earth metals, Re, and iron (ReFe.sub.2) and a eutectic phase of Dy and ReFe.sub.2. The presence of these phases counter the ductility of the metal phase, compromising the toughness of the product.
Composites of Terfenol-D and other magnetostrictive materials have been formed with a matrix of a binder metal, such as aluminum, nickel, or iron. These have been made by hot pressing or sintering powdered magnetostrictive materials with an appropriate metal or metal-alloy powder. However, the problem with these composites is similar to that discussed above, i.e., phases form between the Terfenol-D powder particles and the binder material at the conditions of formation. These phases lend brittleness to the composites, resulting in brittle Terfenol-D/metal binder composites with inadequate toughness for many applications. In addition, the amount of magnetostrictive material is reduced because of reaction with the binder material, thus reducing the magnetostriction constant of the composite. In summary, those methods for forming magnetostrictive particle/metal binder composites by hot pressing or sintering result in undesirable phase or phases that compromise the physical properties of the shape.
Composites have also been made of magnetostrictive materials, such as Terfenol-D, in a matrix of a polymeric resin binder material. The composites have good toughness properties. However, a problem with polymer matrix composites is that the stiffness is significantly lower, which limits their frequency response.
A long felt need in the art is method for forming magnetostrictive shapes that have the toughness, and machinability of resin matrix composites with higher strength.